I was told that the way the towers were designed was to have an indestructable frame, and that each floor were hanging within said frame. The way they were forced to collapse was by heating the welds on the top suspended floor to the point of melting, with the jet fuel on fire. Once the first suspended floor fell it overloaded the second then the third etc.
I don't have any proof of any of this, just an engineer friends explanation - and haven't cared much to follow up on it. But by this explanation it would make sense by how it collapsed, and that there wouldn't need to be a lot of damage to force it - just high temperature on the top floor.
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